📚 Price Changes | 价格变化
Price changes are among the most fundamental concepts in economics. In every market, prices rise and fall as the forces of demand and supply interact. Understanding why prices change, how they affect different groups, and what they signal to the wider economy is essential for any IGCSE Economics student preparing for the Cambridge examination.
价格变化是经济学中最基本的概念之一。在每个市场中，价格都随着需求与供给力量的相互作用而上下波动。理解价格为何变化、如何影响不同群体，以及向整个经济传递了什么信号，对于每一位备考剑桥IGCSE经济学考试的学生来说都至关重要。
1. The Market Mechanism: Demand and Supply | 市场机制：需求与供给
The market mechanism, also known as the price mechanism, is the system through which buyers and sellers communicate via prices. The law of demand states that, ceteris paribus, as price increases, the quantity demanded decreases. Conversely, the law of supply states that as price increases, the quantity supplied increases because producers are willing to supply more at higher prices to earn higher profits.
市场机制（又称价格机制）是买卖双方通过价格进行沟通的系统。需求定律指出，在其他条件不变的情况下，当价格上升时，需求量下降。相反，供给定律指出，当价格上升时，供给量增加，因为生产者愿意以更高的价格提供更多商品以获取更高利润。
- The demand curve slopes downward from left to right, showing the inverse relationship between price and quantity demanded.
- The supply curve slopes upward from left to right, showing the positive relationship between price and quantity supplied.
- Price acts as a signal, an incentive, and a rationing tool in allocating scarce resources.
- 需求曲线从左向右向下倾斜，显示价格与需求量之间的反比关系。
- 供给曲线从左向右向上倾斜，显示价格与供给量之间的正比关系。
- 价格在配置稀缺资源时起信号、激励和配给工具的作用。
2. Market Equilibrium | 市场均衡
Equilibrium is the state in which the quantity demanded by consumers exactly equals the quantity supplied by producers. The price at which this occurs is called the equilibrium price, also known as the market-clearing price. At this point, there is no excess demand or excess supply, and the market is said to clear.
均衡是指消费者的需求量恰好等于生产者供给量的状态。这时的价格称为均衡价格，又称市场出清价格。在这一点上，既没有超额需求也没有超额供给，市场实现"出清"。
If the actual price is above the equilibrium price, a surplus exists: producers cannot sell all their output, so they cut prices to attract buyers. If the actual price is below the equilibrium price, a shortage exists: consumers compete to buy limited goods, pushing prices upward. In both cases, market forces drive the price back toward equilibrium.
如果实际价格高于均衡价格，市场上会出现盈余：生产者无法卖出全部产出，因此降低价格以吸引买家。如果实际价格低于均衡价格，市场上会出现短缺：消费者争相购买有限的商品，推高价格。在这两种情况下，市场力量都会推动价格回归均衡。
3. Demand-Side Causes of Price Changes | 需求侧价格变动原因
A change in any factor affecting demand other than price will shift the entire demand curve. A rightward shift indicates an increase in demand, pushing the equilibrium price upward. A leftward shift indicates a decrease in demand, pushing the equilibrium price downward. These shifts are caused by the following factors.
除价格以外的任何影响需求的因素发生变化，都会使整条需求曲线发生移动。需求曲线向右移动表示需求增加，推动均衡价格上升；向左移动表示需求减少，推动均衡价格下降。以下因素会引起这些移动。
- Changes in real household income: rising incomes increase demand for normal goods, shifting the demand curve rightward.
- Changes in tastes and preferences: fashion trends, advertising, or seasonal popularity can boost demand.
- Changes in the price of substitutes and complements: when the price of tea rises, demand for coffee (a substitute) may increase.
- Changes in population size or structure: an ageing or growing population alters demand patterns.
- Expectations of future price changes: if consumers expect prices to rise tomorrow, they buy more today.
- 家庭实际收入变化：收入增加会提高对正常商品的需求，使需求曲线右移。
- 口味和偏好变化：时尚潮流、广告或季节性流行都会推动需求上升。
- 替代品和互补品价格变化：当茶的价格上涨时，对咖啡（替代品）的需求可能会增加。
- 人口规模或结构变化：人口老龄化或人口增长会改变需求模式。
- 对未来价格变化的预期：如果消费者预期明天价格上涨，他们会今天就购买更多。
4. Supply-Side Causes of Price Changes | 供给侧价格变动原因
Similarly, a change in any factor affecting supply other than price will shift the supply curve. A rightward shift represents an increase in supply, which pushes the equilibrium price downward. A leftward shift represents a decrease in supply, which pushes the equilibrium price upward. The main causes include the following.
同样，除价格以外的任何影响供给的因素发生变化，都会使供给曲线发生移动。供给曲线向右移动表示供给增加，推动均衡价格下降；向左移动表示供给减少，推动均衡价格上升。主要原因包括以下内容。
- Changes in costs of production: higher wage costs, energy prices, or raw material prices reduce supply.
- Technological improvements: better machinery and processes lower costs and increase supply.
- Taxation and subsidies: an indirect tax reduces supply, while a subsidy increases supply.
- External shocks: floods, droughts, natural disasters, and wars can severely disrupt supply.
- Changes in the number of suppliers: more firms entering the market expands supply.
- 生产成本变化：工资、能源价格或原材料价格上涨会减少供给。
- 技术进步：更好的机器和生产工艺降低成本并增加供给。
- 税收和补贴：间接税减少供给，补贴增加供给。
- 外部冲击：洪水、干旱、自然灾害和战争会严重扰乱供给。
- 供应商数量变化：更多企业进入市场会扩大供给。
5. Price Elasticity of Demand | 需求价格弹性
Price elasticity of demand (PED) measures the responsiveness of quantity demanded to a change in price. The formula is:
需求价格弹性（PED）衡量需求量对价格变化的反应程度。公式为：
PED = %ΔQD ÷ %ΔP
demand is unitary elastic. This concept is crucial for firms because it determines the effect of a price change on total revenue.
如果PED > 1，需求富有价格弹性，意味着需求量的变化比例大于价格的变化比例。如果PED < 1，
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